The present invention relates to a die casting apparatus.
Most of the conventional die-casting machines have been such that the direction in which a cover die and a rejector die are joined and held securely together is same as the direction in which the molten metal is forced into a cavity. But there have been recently devised and demonstrated die-casting machines of the type in which a cover die and a rejector die are held securely together in the horizontal direction while the molten metal is forced into the cavity from the lower end or bottom thereof, such die-casting machines being referred to as "horizontal die clamping and vertical molten metal pouring type die casting machines" in this specification.
The die-casting machines of the latter type have various advantages. First, the length of molten metal in a pouring or ladling sleeve is short before the molten metal is forced into the cavity so that the temperature drop of molten metal can be minimized. The surface of contact between molten metal and air is small and a lesser quantity of air is entrained in the molten metal when the latter is forced into the cavity so that the die castings have less porosity which results from gases in the pouring sleeve. When the molten metal has been completely filled into the cavity, an injection plunger remains in opposed relationship with the cavity so that the pressure can be effectively transmitted. However, there is the problem that when a molten metal is forced into a cavity, a temperature drop occurs and the molten metal is solidified along the inner wall surface of the pouring sleeve and the solidified metal intrudes into the cavity, resulting in the degradation of the quality of the die castings.
In view of the above, applicant has disclosed a die-casting method and a die-casting machine which can prevent the intrusion of solidified metal into the cavity in Japanese Published Patent No. 58-55895 (1983). In this die-casting machine, a cover die and a rejector die are fitted with two-split stationary sleeves whose lower ends are made into contact with a pouring sleeve which is forced upwardly. A vertically reciprocable plunger is fitted into the pouring or ladle sleeve in such a way that the injection cylinder of a vertical molten metal pouring unit causes vertical reciprocal movement of the plunger. A small-diameter restricted portion intercommunicates between the cavity defined by the cover and rejector dies and the bore of a stationary sleeve.
In operation, after the molten metal has been poured into the pouring sleeve, the latter is forced upward and into contact with the stationary sleeve. Thereafter the plunger is advanced so that the molten metal is forced through the bore of the stationary sleeve and the small-diameter restricted portion into the cavity. When the molten metal is being forced into the cavity, a shell or a thin film cylindrical solidified metal formed along the inner wall surface of the stationary sleeve is corrugated and compressed between the plunger and the stepped surface immediately before the small-diameter restricted portion so that the shell remains in the bore of the stationary sleeve and therefore can be prevented from intruding into the cavity. The die casting which has been ejected out of the cavity after the molten metal has been completely solidified is connected to the so-called "biscuit" of excess metal left above the plunger by a portion of solidified metal corresponding to the small-diameter restricted portion. The biscuit can be easily separated from the die casting by breaking the fine solidified metal portion corresponding to the small diameter restricted portion.
However, in the die casting apparatus of the type described above, when the pouring sleeve and the plunger are moved downward after the molten metal has been forced into the cavity, the "biscuit" is in intimate contact with the upper end surface of the plunger so that it is also moved downward in unison with the pouring sleeve. As a result, the "biscuit" is broken off from the fine solidified metal portion corresponding to the small-diameter restricted portion so that the die casting is ejected out of the cavity while the "biscuit" remains on the side of the pouring sleeve. According to the partial shot method whose objective is to attain satisfactory casting conditions based upon the observation of the flow of the molten metal in the cavity, the plunger is forced downward at a suitable time when the cavity is partially filled with the molten metal. In this case, the "biscuit" tends to be broken off of the fine solidified metal portion corresponding to the small-diameter restricted portion. Especially when the plunger is stopped in the pouring sleeve, the "biscuit" is always forced to move downward in unison with the pouring sleeve when the length of the portion of the biscuit remaining in the pouring sleeve is longer than the length of the stationary sleeve.
When the biscuit remains on the side of the pouring sleeve in the manner discribed, the new molten metal cannot be ladled into the pouring sleeve. Furthermore when the injection cylinder is caused to be inclined while the biscuit broken off from the fine solidified metal corresponding to the small-diameter restricted portion remains projecting beyond the pouring sleeve, the leading end of the biscuit projecting beyond the pouring or ladling sleeve strikes against the notched rim at the lower end of the stationary platen so that the injection cylinder cannot be inclined as desired. As a result, the portion of the biscuit extending out of the pouring sleeve must be cut off by using gasses and then the pouring sleeve is inclined to the ladle position. Thereafter the plunger is pushed upward so that the biscuit remaining in the pouring or ladle sleeve must be pushed out of it and removed. As a result, the efficiency of the die-casting operation is considerably degraded. Furthermore, with the biscuit extending from the pouring sleeve, when the plunger is forced upward while the pouring sleeve remains at its lowered position so as to push the biscuit out of the pouring sleeve, the leading end of the biscuit engages with the lower end surface of the small-diameter restricted portion of the cover die because the downward stroke of the pouring sleeve is short. It follows therefore that unless the biscuit is cut off by using gases, it cannot be removed out of the pouring sleeve.
Moreover, the conventional stationary sleeve has a completely cylindrical inner wall surface so that the space at which the shell remains is not sufficient in area. Furthermore, the shell has a tendency to move toward the small-diameter restricted portion so that there is a tendency for the shell to intrude into the cavity through the small-diameter restricted portion.
Still further, when molten metal injection has been carried out by using the conventional stationary sleeve having a completely cylindrical inner wall surface and then, if it has happened during injection that a considerably large molten metal piece is poured into the cavity of a mold, smooth flow of the molten metal will suffer interference from such solidified metal piece, thus inadequate filling of the mold cavity with molten metal results therefrom.